I. FIELD OF THE INVENTION
This invention relates to the preparation of strong agglomerates from airborne dusts especially, but not exclusively, alumina-containing dusts such as electrostatic precipitator (ESP) dust derived from a process of converting a material containing predominantly gibbsite to sub-alpha alumina or corundum (alpha alumina), and to the agglomerates thus produced.
II. DISCUSSION OF THE PRIOR ART
Material containing predominantly gibbsite (alumina trihydrate, Al.sub.2 O.sub.3 .multidot.3H.sub.2 O) is produced by the Bayer process and is subsequently thermally converted into sub-alpha alumina (or corundum) of low water content, typically less than 3%, by a multi-stage heating process. Airborne dust is produced during this process and is usually collected by means of electrostatic precipitators in order to avoid environmental pollution. The suspended or airborne dust collected in this way is a very fine undercalcined alumina having a particle size which may vary according to the shape or density of the particles and according to the gas or air velocity. Typically, more than 60% of the dust is in the form of particles less than 20 microns in size and the particles generally have a median size of 3-5 microns. The dust usually consists of a mixture of particles of fully calcined (anhydrous) alumina, partially calcined alumina and uncalcined (trihydrate) alumina which collectively exhibit a loss of mass (LOM) on heating from ambient to 1100.degree. C. (sometimes referred to as loss on ignition or LOI) of between 1 and 35% by weight.
Attempts have been made to utilize such dust by adding it to the bauxite feed of the Bayer digestion process or by recycling the dust to other process feeds or product lines. However, this is not very satisfactory because the dust is difficult to handle and may not contain the proper ingredients for the processes or products involved. For example, ESP dust generally contains 5-80% of a form of alumina which does not undergo caustic digestion when used in the Bayer process and thus ends up in the "red mud" waste product of the process, which itself presents disposal problems. As a result, ESP dust is often just slurried and pumped to a disposal site.
Dusts of this kind, and dusts produced by other industrial processes, e.g. dusts produced by the treatment of various other minerals and ores, would be much easier to handle if they could be agglomerated into particles of larger size since they would then have reduced tendencies to become airborne. This could be achieved by the use of suitable binders (e.g. various polymers, etc.) followed by pelletization or the like, but polymeric binders tend to be expensive and remain in the product as contaminants, making the agglomerated product unsuitable for recycling to various processes, and would be eliminated at the high temperatures used in various treatment processes, leading to easy fracture of the particles and regeneration of the original dust.
Several processes are already known for the agglomeration of particles of alumina but these involve complex and expensive steps and generally do not relate to the treatment of dusts, i.e. particles of such a small size that the particles easily become airborne.
For example, U.S. Pat. No. 4,169,874 to Bambrick teaches that shaped alumina particles can be produced by preparing an aqueous slurry of an alumina composition containing a substantial portion of rehydratable alumina, and then subjecting the slurry to shaping, rehydrating and curing. The shaped bodies are formed by first making a slurry having a solids content from 50-60% and passing it through an immiscible phase at a temperature from 80.degree.-100.degree. C.
Furthermore, U.S. Pat. No. 4,579,839 to Pearson discloses a process in which flash activated alumina is used to bond ceramic bodies together. Agglomeration and shaping is then carried out in a hot liquid which is immiscible in water.
However, as stated above, such processes are not well suited to the treatment of dusts and tend to be too expensive for application to waste materials.